1. Field of the Invention
This invention relates to sulfited fatty compounds with a reduced content of free hydrogen sulfite, to a process for their production, in which unsaturated fatty compounds are blown and sulfited and unreacted hydrogen sulfite is fixed as sulfosuccinates by aftertreatment with maleic acid esters and optionally bases, and to the use of the sulfited fatty compounds as leather auxiliaries.
2. Discussion of Related Art
Apart from tanning agents, oiling agents are the most important auxiliaries for bringing out the character of leather. Oiling agents develop their effect by fiber-insulating lubrication and by hydrophobicization.
The coating of the leather fibers with a fatty film reduces mutual friction and, hence, improves the suppleness and elasticity of the tissue. This has positive effects on the tear strength of the leather because, in an elastic material, many fibers--when subjected to tensile stressing--align themselves in the same direction as the tensile stress and then offer greater resistance to tearing than the same fibers in a brittle material. In addition, tanning effects are obtained through the hydrophobicization because hydrophobicization is accompanied by the displacement of water from the skin.
Vegetable and animal oils, fats and waxes and the hydrolysis, sulfonation, oxidation and hydrogenation products obtained therefrom by chemical conversion and, finally, mineral oiling agents are generally used for oiling leather. More specifically:
Saponifiable fats and oils and natural waxes and resins belong to the esters. Oils and fats are understood by the leather expert to be esters of glycerol and fatty acids which are solid or liquid at room temperature. From the group of animal fats, train oils, fish oil, beef tallow and neat's foot oil in particular and, from the group of vegetable fats, castor oil, rapeseed oil and linseed oil are used for oiling leather. In waxes and resins, the fatty acids are esterified with relatively high molecular weight alcohols instead of glycerol. Examples of waxes are beeswax, chinese wax, carnauba wax, montan wax and wool grease. The most important resins include colophony, birch bark oil and shellac.
The chemical conversion of vegetable and animal fats gives products which are soluble in water and which, in addition, emulsify water-insoluble fatty compounds to different extents. Known products include, for example, the sulfonated water-soluble oils of various kinds, train oils modified by oxidation which are known as degras or moellon, the soaps formed in the hydrolysis of natural fats, hydrogenated fats and, finally, free fatty acids, such as stearic acid, as baking fats. Most animal and vegetable fats have a certain affinity for leather which is considerably enhanced by the introduction or exposure of hydrophilic groups.
Mineral oiling agents are also important in leather manufacture. These hydrocarbons are similar to natural fats and oils in certain properties, but cannot be saponified. They are fractions from the distillation of petroleum which are known as mineral oil in liquid form, as vaseline in paste-like form and as paraffin in solid form.
In many cases, however, unwanted stains known as fatty spew are formed over a period of time on the surface of tanned and oiled leather. Fatty spew is mainly formed on chrome-tanned leather after relatively short or prolonged storage as a white, often bloom-like coating which only covers individual parts of the leather surface or even the entire leather surface. Fatty spew is attributable to the secretion of solid fatty compounds from the leather. It can be caused by the natural fat present in the leather or by fatty compounds which have been incorporated in the leather during the oiling process.
Fatty mixtures used to oil leather tend to form fatty spew in particular when they contain many free fatty acids. Free fatty acids generally have a higher melting point than their glycerides. The hydrolysis of fatty compounds during storage of the leather increases the risk of fatty spew accordingly.
Soaps and fat liquors are hydrolyzed in chrome leather, especially in inadequately deacidified chrome leather, with release of fatty acids. Sulfonated oils and fats differ in their tendency to form fatty spew, the tendency to form fatty spew generally diminishing with the age of the leather [cf. J. Int. Soc. Leath. Trad. Chem. 379 (1952)].
Fatty spew is formed more easily, the greater the content of fatty compounds tending to form fatty spew in the leather. The quantity, composition and position of the fatty mixture of natural fat and fat liquor present in the leather are critical to the extent and composition of the fatty spew. Loosely structured leather tends to form fatty spew less than leather with a dense fiber structure. Fatty spew is observed more often at low temperatures than at relatively warm outside temperatures.
The crystalline fatty spew forms in the follicles and glandular channels, small crystals initially being formed deep inside the follicle and gradually filling the entire follicle as relatively large fatty crystals which then spread out over the surface of the leather and coalesce into a dense crystal film. Any fats containing stearin or palmitin derivatives can cause crystalline fatty spew, the danger of fatty spew increasing with increasing concentration [Ledertechn. Rundsch. 1 (1949)].
So-called neutral fats in particular, i.e. substances with no ionic groups in the molecule which are suitable for oiling leather, for example fats, waxes and hydrocarbons, tend to form fatty spew. Those neutral fats which are stearin and/or palmitin derivatives, such as for example corresponding triglycerides or the free fatty acids, are particularly critical in this regard.
Since oiling is virtually a compulsory step in the processing of leather, but after tanning, to establish the required product properties, it has become standard practice to use special synthetic oiling compositions with only a minimal tendency to form fatty spew.
One class of oiling agents widely used for this purpose are halogenated compounds, such as chlorinated hydrocarbons. Unfortunately, the increasing ecological and toxicological requirements which compositions entering the environment or coming into contact with the consumer are expected to satisfy make this class of compounds increasingly unattractive. The use of chloroparaffins as additives to fat liquor emulsions to prevent the formation of fatty spew on chrome-tanned pigskin is described, for example, by J. Golonka in Przegl. Skorzany 42(2), 35 (cf. Chem. Abstracts 107(18):156865z).
EP-B 0 247 509 (Stockhausen) describes adducts of sulfuric acid or oleum with unsaturated, alkoxylated and optionally epoxidized fats and oils. Unfortunately, products of this type generally have an unfavorably high electrolyte content so that the salts are in danger of crystallizing out and impairing the quality of the treated leather.
In addition to reactions with oleum, sulfuric acid or gaseous sulfur trioxide, sulfiting i.e. the addition of hydrogen sulfite onto unsaturated compounds, is particularly suitable for the hydrophilicization of fatty compounds. For example, A. Kuntzel reports in this connection on the sulfiting of cod liver oil [Leder 8, 5 (1957)] while M. Mikula reports on the sulfiting of unsaturated fatty acid butyl esters [Leder, Schuh, Lederwaren 21, 282 (1986)]. Although sulfited fatty compounds show particularly favorable oiling behavior, they also tend to form unwanted fatty spew. Another disadvantage is that the products have a pungent odor attributable to small amounts of unreacted hydrogen sulfite.
Accordingly, the methods from the prior art for preventing fatty spew are not altogether satisfactory.
It is clear from the above context that there is a constant need in the leather industry for additives or oiling compositions which effectively prevent fatty spew so that the range of commercial products can be extended to allow a flexible response to the changing requirements of the market. More particularly, there is a need for ecologically or rather toxicologically safe additives and oiling compositions of which the use does not result in the unwanted formation of fatty spew.
Accordingly, the problem addressed by the present invention was to provide new substances for oiling and hydrophobicizing leather which would be free from the disadvantages mentioned above.